Validation & Structure Quality

1. Validation & Structure Quality

2. Ground rules for Bioinformatics • Don't always believe what programs tell you they're often misleading & sometimes wrong! • Don't always believe what databases tell you they're often misleading & sometimes wrong! • Don't always believe what lecturers tell you they're often misleading & sometimes wrong! • In short, don't be a naive user • when computers are applied to biology, it is vital to understand the difference between mathematical & biological significance • computers don’t do biology - they do sums quickly!

3. Validation • 1: the act of validating; finding or testing the truth of something • 2: the cognitive process of establishing a valid proof • Assessing the quality of a model is called validation. Validation is something that needs to be done both by producers (crystallographers, NMR spectroscopists, electron microscopists, etc.) and users (biologists, enzymologists, medicinal chemists, etc.) of models.

4. Some Truths • Never trust a structure at face value. • Any structure is only as good as the experimental data which goes into its determination. • Just because it is published in Nature/Cell/Science does not mean the structure is not without flaws.

5. Errors in Structures • Completely wrong • Wrong trace, incorrect fold of protein • Register errors, where trace of protein is not in keeping with sequence order. • Partial errors • Incorrectly built loops. • Wrong residues built into the structure (i.e., Proline instead of Aspartic acid). • Bad data quality • Bad geometry and stereochemistry. • Incorrect positioning of ligands etc due to lack of experimental evidence. • FRAUD !!

6. Some Quality Indicators Some data quality indicators for structures are • Ramachandran Plot • Geometry and Stereochemistry • R-factor/FreeR-factor (Structures from X-ray crystallography) • Correlation between experimental data and structure • Resolution of the data upon which the structure is based (Structures from X-ray crystallography)

7. Ramachandran Plot • A graph between the dihedral angles of an amino acid in a protein. • Due to steric hindrance from amino acid side chains, only certain angles are allowed in a folded protein. • A plot between the dihedral angles of individual amino acids in a protein can serve to indicate how well the structure has been determined. • Any deviations from the allowed values are called Outliers and usually indicate bad geometry

8. Ramachandran Plot Standard Plot showing where different secondary structures fit into the plot. A real life example. All non-glycine residues are in allowed regions.

9. Validation • Ideally, there should be no outliers in the Ramachandran plot, except for Glycine and Proline, which are “special” amino acids. • However, there may be some rational explanation for outliers by the scientist depositing the structure. (Always refer to the publication!). • Expect to find more than 85-90% of residues to fall into the red regions.

10. Geometry and Stereochemistry This is supposed to be Phenylalanine and should look like: BUT….

11. Geometry and Stereochemistry This is supposed to be a sugar and should look like: BUT….

12. Geometry and Stereochemistry This is supposed to be another sugar (sucrose) and should look like: BUT….

13. Geometry and Stereochemistry Always look at the structure in graphical viewers. Look at the geometry section in PDB files (REMARK 500). Use tools like PDBeAnalysis, PDBSum to analyze structures. http://www.ebi.ac.uk/pdbe-as/PDBeValidate

14. R-Factor/Correlation R-factor is a measure of the agreement between the crystallographic model and the experimental X-ray diffraction data. Free R-factor is calculated between the structure and a certain subset of the data excluded from the structure calculation process. In a good structure, the difference between R-factor and Free R-factor (DR) should be less than 5%. Correlation calculates the overall correlation between the structure and the data available. Good structure should have overall correlation in excess of 90%. See PDBe atlas pages for experimental correlations in crystal structures Look at the R-factors on the Atlas Pages in the tutorials !!!

15. Resolution Resolution is a indicator of the level of detail available in the data used for determining structures in X-ray crystallography. Higher resolution (lower number) means that there is more detail available. Low resolution: <3.0A Medium resolution: 1.8-3.0A High Resolution: 1.0 – 1.8A Atomic Resolution: >1.0A Not all parts of the structure are at the same resolution…

16. So what do you look for… Higher resolution structures where more than one available Good geometry and stereochemistry (Look at the Ramachandran plot) Lower R-factor and DR (FreeR-factor – Rfactor) High correlation coefficient between experimental data and structure. Complete structures (pay attention to the Sequence and how much of it is represented in the structure), with no sequence conflicts. Structures with ligands bound may be more useful for analysis than apo-form structures. Note: These are general guidelines which may help you choose the best structure for your analysis where more than one structure for the same protein is available.

17. General Evaluation Criteria Be sceptical and cynical! • When you are searching for information you need to judge its quality and suitability. • Think critically about each piece of information you find and how you found it. • Relevance: • Does the information you have found adequately support your research? • Does it answer the question, or support one of your arguments? • How general or specific is the information about the topic?

18. Some programs for Structure Validation: Procheck http://www.biochem.ucl.ac.uk/~roman/procheck/procheck.html WHATCHECK: http://swift.cmbi.ru.nl/gv/whatcheck/ JCSG Validation: http://www.jcsg.org/scripts/prod/validation1.cgi PDBeAnalysis: http://www.ebi.ac.uk/pdbe-as/PDBeValidate Validation

19. Wrong Structures !! PDB entry 1PHY PDB entry 2PHY

20. Wrong Structures PDB entry 1PTE PDB entry 3PTE

21. “were incorrect in both the hand of the structure and the topology. Thus, the biological interpretations based on the inverted models for MsbA are invalid.” 1PF4

22. “However, because of the lack of clear and continuous electron density for the peptide in the complex structure, the paper is being retracted.” 1F83

23. UAB Researcher involved in fraud ! After a thorough examination of the available data, which included a re-analysis of each structure alleged to have been fabricated, the committee found a preponderance of evidence that structures 1BEF, 1CMW, 1DF9/2QID, 1G40, 1G44, 1L6L, 2OU1, 1RID, 1Y8E, 2A01, and 2HR0 were more likely than not falsified and/or fabricated and recommended that they be removed from the public record. The former employee was H.M. Krishna Murthy, who was found by the Investigation Committee to be solely responsible for the fraudulent data. The coordinates for 2HR0 do not form a connected network of molecules in the crystal lattice. 

24. What do you get as a Structural Biologist? • The best chances of winning a Nobel Prize (1946, 1962, 1962, 1972, 1982, 1988, 1991, 1997, 2002, 2003, 2006, 2009), all in X-ray crystallography except 2002. • Islands in the Antarctic: • Perutz Glacier 67° 37' S, 66° 25W. • Bragg Islands 66° 28' S, 66° 27' W. • Shull Rocks 66° 27' S, 66° 40' W. • Pauling Islands • Bernal Islands